The invention relates to a method for controlling a sheet-making process and to a control arrangement of the sheet-making process operating according to the method.
The quality of a sheet being manufactured is usually measured across the web by cross-directional (CD) and other measurements. Typical variables measured in CD measurements are moisture content, caliper, basis weight, and ash content. Other typical measurements which describe the quality of the sheet are color, opacity, gloss, and smoothness, which may or may not be CD measurements. The measurement results are compared with the set values and an error profile is formed to show the difference between the measurement results and the set values. A process which is in a state matching the set values is known to produce a sheet of a desired quality and, thus, the process should be kept in a state matching the set values as exactly as possible. By means of the error profile and a nominal process model, a controller gives a control command to one or more actuators which alter the process according to the command. The process alteration results in a change in the CD or other properties of the sheet, and this change occurs in the time domain (TD) in a certain way. The nominal process model describes the TD and/or CD and/or other aspects of the process response to the actuator. In paper-making in particular, amount or combinatory proportion of material or mechanical load or heat applied to the sheet during the process can be used as actuators. The measurement variables are altered or maintained by means of the actuators to keep the quality of the paper being made good. For each combination of actuator and measured sheet property, the process model comprises some or all of (i) CD mapping, (ii) CD response, (iii) TD mapping, (iv) TD response, (v) process gain, and may comprise additional aspects as well. One such sheet-making system is described in U.S. Pat. No. 5,539,634, which is incorporated herein by reference. The terms mapping and response are known per se to those skilled in the art of control of sheet properties, but are defined later in this disclosure.
However, there is always some uncertainty in the process model used by a controller. A sophisticated controller may perform very well with an accurate process model, but the disadvantage is that the controller performs badly with an inaccurate model. The deterioration in control performance depends on the nature of the inaccuracy in the model and the sensitivity of the controller to that kind of error.
Thus, it is an object of the invention to implement a method and a control arrangement implementing the method in such a manner that the process can also be controlled well with an inaccurate nominal process model. The method and the arrangement therefore comprise a robust controller. This object is achieved by the method for controlling the sheet-making process disclosed in the preamble, in which method a sheet matching the desired values is manufactured, in which method: the state of the process is measured to get one or more measurement values representing the state of the process; each measurement value is compared with a corresponding desired value; on the basis of the comparison an error is formed, representing at least one difference between the measurement value and the desired value; at least two control action candidates based on the error are formed by means of at least two process models, of which at least one is a perturbed process model; one control action is formed from the control action candidates to control the sheet-making process to produce a sheet matching the desired values.
A further object of the invention is a control arrangement for controlling a sheet-making process which is adapted to produce a sheet matching the desired values, which arrangement comprises: a measuring unit for measuring the state of the process, which produces one or more measurement value representing the state of the process; a comparator for comparing each measurement value with the corresponding desired value and for forming an error representing at least one difference between the measurement value and the desired value; a control unit which is adapted to utilize at least two process models, of which at least one is a perturbed process model, the control unit being also adapted to form at least two control action candidates based on the error; a processing unit for forming one control action of the control action candidates, with which control action the control arrangement is adapted to control the sheet-making process to produce a sheet matching the desired values.
Preferred embodiments of the invention are set forth in the dependent claims.
The method and arrangement of the invention provide several advantages. One major advantage is that a robust controller tolerates well errors in the process models and thus, it performs well, even with errors (of limited magnitude) in its process model. The controller performs well because its actions have been formed from a range of process models, which have a higher likelihood of encompassing the true process model than would a single nominal model.